This application is based on Patent Application No. 2001-017943 filed Jan. 26, 2001 in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to an interferometer and its fabrication method, which enable planar optical waveguide circuits used in optical communication field to adjust optical path lengths (phases) independently in the transverse electric (TE)polarization mode and transverse magnetic (TM) polarization mode.
2. Description of the Related Art
Optical circuits employing single mode waveguides formed on a substrate are characterized by high integration and mass productivity, and hence they are essential to construct economical optical network nodes. In particular, optical circuits utilizing silica-based waveguides with SiO2 as the main ingredient has favorable characteristics such as low loss, superiority in an affinity for silica-based optical fibers, and long-term stability. Thus, a large variety of optical components typified by arrayed waveguide gratings are put to practical use, and are applied to commercial systems.
These optical components are fabricated by combining a glass film deposition technique such as flame hydrolysis deposition (FHD) and chemical vapor deposition (CVD) with a microfabrication technique such as reactive ion etching (RIE). More specifically, a glass film is deposited on a substrate such as a silicon wafer to form an lower-cladding, followed by depositing a core layer with a refractive index higher than that of the cladding layer. Then, a core pattern is formed by the microfabrication technique to form an optical circuit, followed by depositing a glass film to form an over-cladding layer, thereby fabricating an optical circuit composed of embedded waveguides.
Usually, the FHD carries out annealing with high temperature to consolidate a glass film, and the CVD also performs annealing to increase the transparency of a glass film. The high temperature process causes thermal stress in the glass film constituting the waveguides, resulting in waveguide birefringence (B-value) in which the effective refractive index of the waveguide varies depending on the polarization state, thereby bringing about optical polarization dependency in the circuit characteristics. In addition, since the waveguide birefringence differs slightly in a wafer surface because of fabrication error, it is necessary to trim the waveguide birefringence locally for each optical circuit to achieve satisfactory optical circuit characteristics.
As a conventional local waveguide birefringence trimming technique, a method is proposed that utilizes a stress-applying film consisting of an amorphous silicon thin film (Japanese Patent Application Laying-open No. 1-77002 (1989). It exploits a phenomenon that an amorphous silicon thin film, which is placed on a waveguide, causes a strong tensile stress in the waveguide, thereby varying the effective refractive index of the waveguide through the photoelastic effect of the glass. Varying the profile of the amorphous silicon thin film enables the control of the stress distribution, that is, the waveguide birefringence. Besides, since the amorphous silicon thin film can be removed by an Ar laser or the like, fine trimming of the length of the amorphous silicon thin film in accordance with the optical circuit characteristics enables the effective refractive index of the waveguide to be adjusted including the waveguide birefringence.
The technique using the amorphous silicon stress-applying film is more actively applied to a constituent element of a polarization beam splitter (PBS) (for example, see, xe2x80x9cBirefringence control of silica waveguides on Si and its application to a polarization-beam splitter/switchxe2x80x9d, Journal of Lightwave Technology, Vol. 12, No. 4, Apr. 1994).
The PBS comprises a Mach-Zehnder interferometer (MZI), which includes two 3 dB optical couplers (50% optical couplers) consisting of silica-based waveguides and two waveguide arms formed on a substrate (silicon substrate), and on which three types of amorphous silicon stress-applying films with different width are placed.
One of the three types of the amorphous silicon stress-applying films with different width is 50 xcexcm wide, and is provided to control the waveguide birefringence principally. The remaining two types are 90 xcexcm and 100 xcexcm wide, and are basically provided to control the effective refractive index of the waveguide polarization independently. The length of the amorphous silicon stress-applying film is trimmed by removing its part by an Ar laser, so that the optical path length difference between the two waveguide arms becomes zero for the transverse magnetic polarization mode, and xcex/2 for the transverse electric polarization mode, where xcex is the wavelength. Thus, according to a known interference principle, the transverse magnetic polarization mode of the light entering the input port is guided to the cross port, whereas the transverse electric polarization mode is guided to the bar port. Thus, the MZI functions as a PBS.
The waveguide birefringence trimming technique utilizing the amorphous silicon stress-applying film, however, has a problem of complicating the device configuration and increasing its cost because of the final trimming using a laser and of the need for aligning the position of the laser irradiation at an accuracy of a few tens of micrometers.
On the other hand, a local-heat trimming method (see, Japanese Patent Application Laying-open No. 3-267902 (1991), for example) is put into practice as a method of trimming the effective refractive index of a waveguide. This technique changes the effective refractive index of the waveguide permanently by annealing the waveguide at rather high power by using thin film heaters patterned on the waveguide, thereby trimming the optical path length (phase) of the optical circuit. Since the thin film heaters are formed by the microfabrication technique using a photomask, it is enough to flow current through the thin film heaters without the high accuracy alignment at the annealing. Thus, the trimming is carried out by a rather simple equipment, enabling its automatization rather easily. This method, however, is insufficient as a method of controlling the waveguide birefringence because the principle of the effective refractive index change and the control of the polarization dependency still remain to be elucidated.
The inventors of the present invention have pursued intensive research to find that the principle of the foregoing local-heat trimming method is that it mainly changes the quality of the cladding between the heaters and the core by the local annealing (heating), and particularly the cladding immediately under the heaters (in other words, the glass quality near the top surface of the cladding), thereby causing a stress to be applied on the waveguide. Then, we demonstrated experimentally that the polarization dependency was controllable substantially by changing the stress distribution by adjusting the width w of the local annealing (heating) region. More specifically, we found that when the width of the local annealing region was 1.4-2.6 times the distance d from the top surface of the over-cladding to the core center, that is, in a range of xc2x130% of wo, where wo was twice that distance d, the effective refractive index changed almost polarization independently, and that a local annealing region wider than wo made the transverse magnetic polarization mode more dominant in the refractive index change, whereas a local annealing region narrower than wo made the transverse electric polarization mode more dominant.
Thus, making the width of the local annealing region wider or narrower than wo, twice the distance from the top surface of the over-cladding to the core center, enables the permanent effective refractive index control of the optical waveguide with retaining the polarization dependency. In particular, the local annealing using at least two types of widths makes it possible to achieve the permanent effective refractive index control, that is, birefringence index control with ensuring the complete independence between the transverse electric polarization mode and transverse magnetic polarization mode.
Since the polarization dependency of the effective refractive index is determined by the stress distribution caused by the local annealing, using at least two types of local annealing with different stress distribution principally enables the permanent effective refractive index control or the birefringence index control with securing the complete independence between the transverse electric polarization mode and the transverse magnetic polarization mode. Accordingly, similar effect can be produced by utilizing the difference, for example, in the distance between the local annealing region and the waveguide center, or in the structure such as geometry of the local annealing region in addition to the width of the local annealing region. Furthermore, the stress geometry can be changed by forming a trench by removing part of the cladding near the local annealing region, and by changing its position, depth, etc.
Since the optical path length trimming by the local annealing is only for the purpose of fine adjustment, when a delay difference (optical path length difference) is required in the circuit design, it is preferable to provide a fixed delay optical circuit in advance, and to carry out the trimming in the final stage to improve the characteristics.
As for the annealing means, although a method using the thin film heater is preferable considering the device cost, this is not essential. Any means that can perform the local annealing of the cladding are applicable. For example, a local annealing (heating) means such as a CO2 laser can be utilized.
In summary, the present invention is based on the following new findings. That is, the local annealing can permanently change the refractive index of the optical waveguide consisting of a core and a cladding because it can cause a stress to be applied on the core because of the changes in the quality in the heated portions. Therefore, changing the width of the region to be transformed by annealing (heating), or the distance or position thereof with respect to the waveguide can make the refractive index of the transverse electric polarization mode greater than, or smaller than, or equal to that of the transverse magnetic polarization mode.
According to the findings above, the interferometer and its fabrication method in accordance with the present invention are characterized in that the refractive index or the optical path length is adjusted by annealing the waveguide with at least two types of annealing regions having effect on the polarization.
For example, employing a combination of a first annealing region, which changes the refractive index of the transverse electric polarization mode more dominantly than that of the transverse magnetic polarization mode, and a second annealing region, which equally changes the refractive index of the transverse electric polarization mode and that of the transverse magnetic polarization mode, enables the phase difference of the transverse electric polarization mode to be adjusted to xcex/2 and that of the transverse magnetic polarization mode to zero in a single interferometer, by establishing the optical path length difference between the transverse electric polarization mode and the transverse magnetic polarization mode at a desired value (xcex/2, for example) by the first annealing region, and then by shifting the optical path length difference of the two polarization modes by the same amount.
On the basis of the foregoing findings, according to a first aspect of the present invention, there is provided an interferometer using an optical waveguide, which is formed by embedding a core that has a refractive index higher than that of a cladding into the cladding on a substrate, the interferometer comprising: at least two types of annealing regions that are provided near the optical waveguide, wherein an optical path length of the optical waveguide is trimmed by partially changing an effective refractive index of the optical waveguide by applying annealing to the annealing regions.
Here, as for the structure of the annealing regions: the annealing regions may differ in their width; at least one of the annealing regions may have a width equal to or greater than 2.6 times a distance d from the core center to the top surface of the cladding, or at least one of the annealing regions may have a width equal to or less than 1.4 times the distance from the core center to the top surface of the cladding; the annealing regions may differ in their distances from the optical waveguide to the annealing regions; the annealing regions may differ in presence or absence of a slit formed in the annealing regions in an optical waveguide direction, or in slit width; the annealing regions may differ in presence or absence of a trench formed by partially removing a cladding around the optical waveguide, or in distance from the optical waveguide to trenches, or in depth of the trenches; the interferometer may further comprise fixed delay means for providing delay dependent on a polarization state; and the interferometer may comprise at least one optical coupler and a plurality of optical waveguides connected to the optical coupler.
The interferometer may comprise two 2xc3x972 optical couplers and two optical waveguides connecting the optical couplers, wherein optical path length difference (delay difference) between the two optical waveguides may be trimmed by local annealing (heating) such that it may be an odd multiple of xcex/2 for a transverse electric polarization mode and an even multiple of xcex/2 for a transverse magnetic polarization mode, where xcex is a wavelength, or that it may be an even multiple of xcex/2 for the transverse electric polarization mode and an odd multiple of xcex/2 for the transverse magnetic polarization mode; at least one of the two optical waveguides connecting the two 2xc3x972 optical couplers may comprise polarization dependent fixed delay means; and the local annealing regions may consist of a thin film heater formed on the optical waveguide.
As for the interferometer in which the independent effective refractive index control is not necessary for the transverse electric polarization mode and for the transverse magnetic polarization mode, and desired optical characteristics are achieved by polarization independent trimming, according to a second aspect of the present invention, there is provided an interferometer using an optical waveguide, which is formed by embedding a core that has a refractive index higher than that of a cladding into the cladding on a substrate, the interferometer comprising: one type of annealing region that has a width from 1.4 to 2.6 times a distance from the optical waveguide core center to a top surface of the cladding in a neighborhood of the optical waveguide, wherein an optical path length of the optical waveguide is trimmed by changing an effective refractive index of the optical waveguide by applying annealing to the annealing region.
According to a third aspect of the present invention, there is provided an interferometer fabrication method, wherein local annealing regions utilize thin film heaters formed on the optical waveguide.
As described above, the interferometer in accordance with the present invention can adjust the birefringence index by changing the film quality of the cladding by locally annealing it to cause the stress change by the changing.
In addition, the interferometer may include at least two types of local annealing regions with a stress distribution inducing structure. Accordingly, it has at least two independent trimming parameters with different polarization dependency of the permanent effective refractive index changes. As a result, it can perform completely independent birefringence index trimming for the transverse electric polarization mode and for the transverse magnetic polarization mode.
When the width of the local annealing (heating) region is 1.4-2.6 times the distance from the top surface of the over-cladding to the core center, the change in the effective refractive index is approximately independent of the polarization state. Accordingly, the one type of local annealing region can adjust the optical path length (phase) in a polarization independent manner.
In particular, using the thin film heaters in the local annealing obviates the need for high accuracy alignment of the irradiation position at the local annealing. Thus, simply supplying current to a predetermined thin film heater is enough to achieve the trimming, thereby enabling the trimming using a rather simple device, which is very advantageous in putting the device in accordance with the present invention into practical use.
Moreover, applying the present invention to various types of interferometers such as a polarization beam splitter enables the interference optical path length to be adjusted for each polarization mode, thereby making it possible to achieve high optical characteristics without the phase control using the thermo-optic effect. Thus, the present invention is very advantageous in terms of the power saving of the interferometer.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.